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DATA

LINK

LAYER

TOPIC: Error control & Flow control

Submitted to : Submitted by:

Prof Manraj Singh Pattar li!ha "or#al

$ancy %ain

$i ea %ain

Sharuti %ain

I$'E(

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S no To#ic Page no) Introduction to data lin* layer +, Ser ice! #ro ided to networ* layer -+ Ser ice! .- Framing /0 Error detection 1. Error correction )0/ Flow control ).1 Error control )22 3eference! ,-

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INTRODUCTION

Data Link Layer Design Issues

The data lin* layer ha! a number of !#ecific function! it can carry out4 The!e function!include

)4 Pro iding a well5defined !er ice interface to the networ* layer4

,4 'ealing with tran!mi!!ion error!4

+4 3egulating the flow of data !o that !low recei er! are not !wam#ed by fa!t !ender!4

To accom#li!h the!e goal!6 the data lin* layer ta*e! the #ac*et! it get! from the networ* layer

and enca#!ulate! them into frame! for tran!mi!!ion4 Each frame contain! a frame header6 a

#ayload field for holding the #ac*et6 and a frame trailer4

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Services Provided to t e Net!ork Layer

The function of the data lin* layer i! to #ro ide !er ice! to the networ* layer4 The #rinci#al

!er ice i! tran!ferring data from the networ* layer on the !ource machine to the networ* layer

on the de!tination machine4 On the !ource machine i! an entity6 call it a #roce!!6 in the

networ* layer that hand! !ome bit! to the data lin* layer for tran!mi!!ion to the de!tination4

The job of the data lin* layer i! to tran!mit the bit! to the de!tination machine !o they can be

handed o er to the networ* layer there4

)4 7nac*nowledged connectionle!! !er ice4

,4 c*nowledged connectionle!! !er ice4

+4 c*nowledged connection5oriented !er ice

7nac*nowledged Connectionle!! Ser ice

7nac*nowledged connectionle!! !er ice con!i!t! of ha ing the !ource machine !end

inde#endent frame! to the de!tination machine without ha ing the de!tination machine

ac*nowledge them4 $o logical connection i! e!tabli!hed beforehand or relea!ed afterward4 If a

frame i! lo!t due to noi!e on the line6 no attem#t i! made to detect the lo!! or reco er from it

in the data lin* layer4 Thi! cla!! of !er ice i! a##ro#riate when the error rate i! ery low !o

that reco ery i! left to higher layer!4 It i! al!o a##ro#riate for real5time traffic6 !uch a! oice6

in which late data are wor!e than bad data4 Mo!t 8 $! u!e unac*nowledged connectionle!!

!er ice in the data lin* layer4

c*nowledged Connectionle!! !er ice

9hen thi! !er ice i! offered6 there are !till no logical connection! u!ed6 but each frame !ent i!

indi idually ac*nowledged4 In thi! way6 the !ender *now! whether a frame ha! arri ed

correctly4 If it ha! not arri ed within a !#ecified time inter al6 it can be !ent again4 Thi!

!er ice i! u!eful o er unreliable channel!6 !uch a! wirele!! !y!tem!4

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Connection Oriented Service

9ith thi! !er ice6 the !ource and de!tination machine! e!tabli!h a connection before any data

are tran!ferred4 Each frame !ent o er the connection i! numbered6 and the data lin* layer

guarantee! that each frame !ent i! indeed recei ed4 Furthermore6 it guarantee! that each frame

i! recei ed e actly once and that all frame! are recei ed in the right order4 9ith

connectionle!! !er ice6 in contra!t6 it i! concei able that a lo!t ac*nowledgement cau!e! a

#ac*et to be !ent !e eral time! and thu! recei ed !e eral time!4 Connection5oriented !er ice6

in contra!t6 #ro ide! the networ* layer #roce!!e! with the e;ui alent of a reliable bit !tream4

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SERVICES

)4 Framing

,4 Error control

+4 Flow control

-4 cce!! Control

04 Phy!ical ddre!!ing

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"ra#ing

To #ro ide !er ice to the networ* layer6 the data lin* layer mu!t u!e the !er ice #ro ided to it

by the #hy!ical layer4 9hat the #hy!ical layer doe! i! acce#t a raw bit !tream and attem#t to

deli er it to the de!tination4 Thi! bit !tream i! not guaranteed to be error free4 The number of

bit! recei ed may be le!! than6 e;ual to6 or more than the number of bit! tran!mitted6 and they

may ha e different alue!4 It i! u# to the data lin* layer to detect and6 if nece!!a ry6 correct

error!4

The u!ual a##roach i! for the data lin* layer to brea* the bit !tream u# into di!crete frame!

and com#ute the chec*!um for each frame4

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Error Contro$

?a ing !ol ed the #roblem of mar*ing the !tart and end of each frame6 we come to the ne t

#roblem: how to ma*e !ure all frame! are e entually deli ered to the networ* layer at the

de!tination and in the #ro#er order4 Su##o!e that the !ender ju!t *e#t out#utting frame!

without regard to whether they were arri ing #ro#erly4 Thi! might be fine for

unac*nowledged connectionle!! !er ice6 but would mo!t certainly not be fine for reliable6

connection5oriented !er ice4

The u!ual way to en!ure reliable deli ery i! to #ro ide the !ender with !ome feedbac* about

what i! ha##ening at the other end of the line4 Ty#ically6 the #rotocol call! for the recei er to

!end bac* !#ecial control frame! bearing #o!iti e or negati e ac*nowledgement! about theincoming frame!4 If the !ender recei e! a #o!iti e ac*nowledgement about a frame6 it *now!

the frame ha! arri ed !afely4 On the other hand6 a negati e ac*nowledgement mean! that

!omething ha! gone wrong6 and the frame mu!t be tran!mitted again4

n additional com#lication come! from the #o!!ibility that hardware trouble! may cau!e a

frame to ani!h com#letely

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Error Detection

9hen data i! being tran!mitted from one machine to another6 it may #o!!ible that data

become corru#ted on it! way4 Some of the bit! may be altered6 damaged or lo!t during

tran!mi!!ion4 Such a condition i! *nown a! error4

The error may occur becau!e of noi!e on line6 attenuation and delay di!tortion4 For reliable

communication6 it i! im#ortant that error! are detected and corrected4

Ty#e! of error!

Single bit error

>ur!t error

Single bit error

It mean! only one bit of data unit i! change d from ) to A or from A to )4 Single bit error can

ha##en in #arallel tran!mi!!ion where all the data bit! are tran!mitted u!ing !e#arate wire!4

Single bit error i! lea!t li*ely ty#e of error! in !erial tran!mi!!ion4

>ur!t Error

It mean! two or more bit! in data unit are changed from ) to A or from A to )4 In bur!t error6 it

i! not nece!!ary that only con!ecuti e bit! are changed4 The length of bur!t error i! mea!ured

from fir!t changed bit to la!t changed bit4

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Techniques

Error detection mean! in!#ecting the data !o a! to chec* whether or not data i! changed during

the tran!mi!!ion4 n error detection !y!tem i! needed at the recei er end that !hould chec* the

incoming data and re#ort the number and ty#eB! error! that define ha e occurred4 The mo!t

common techni;ue u!ed for error detection i! redundancy 4

3edundancy

3edundancy i! the method in which !ome e tra bit! are a dded to the data !o a! to

chec* whether the data contain error or not4

The!e redundant bit! are added by the !ender and remo ed by the recei er a! !oon a!the accuracy of the tran!mi!!ion ha! been determined

The!e redundant bit! are then added to the data and data redundant bit! are

tran!mitted to the recei er

Ty#e!

)4 Dertical 3edundancy chec* #arity chec* ,4 8ongitudinal 3edundancy chec*

+4 Cyclic 3edundancy chec*

-4 Chec*!um

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%ertica$ Redundancy C eck&Parity C eck

Thi! method i! al!o *nown a! #arity chec*4 In thi! method6 a redundant bit called #arity bit i!

added to each data unit4 Thi! method may include e en #arity or odd #arity4 E en #arity

mean! that total number of )are e en4 Odd #arity mean! the total no of ) are odd4

E am#le

If !ource want! to tran!mit a data unit ))AA))) u!ing e en #arity to the de!t ination4 The

!ource will fir!t #a!! thi! unit to e en #arity generator4

The #arity generator will count the number of ) in data and will add #arity bit4 In thi!

e am#le6 !ince the number of ) in data unit i! fi e6 the #arity generator a##end! a #arity bit )

to thi! data unit ma*ing the total number of ) e en4

) ) A A ) ) )

D3C can detect all !ingle bit error

It can al!o detect bur!t error but only in tho!e ca!e! where number of bit! changed i!

odd4

The major di!ad antage of thi! method i! that it i! not able to detect bur!t error if the

number! of bit! are e en4

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) ) A A ) ) ) )

E en Parity generator

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Longitudinal Redundancy Check

In thi! method bloc* of bit! i! di ided into table or matri of row! and column!4

In order to detect the error6 a redundant row of bit! i! added to the whole bloc* and thi! bloc* i! tran!mitted to the recei er4

The recei er u!e! thi! redundant row to detect error4 fter chec*ing the dat a for error!6

recei er! acce#t the data and di!card the redundant row of bit!4

9hen thi! data reache! the de!tination6 recei er u!e! 83C to detect error in data4

83C i! u!ed to detect bur!t error!4 For e4g4 !u##o!e +, bit data #lu! 83C that wa! being

tran!mitted i! hit by bur!t error of length fi e and !ome bit! are corru#ted4

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11001010 10101010 11001100 11100011

11001010

10101010

11001100

11100011

01001111

11001010 10101010 11001100 11100011 01001111

Receiver data

Sender data

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Cyc$ic Redundancy C eck

C3C i! more #owerful than D3C and 83C in detecting error!4

It i! not ba!ed on binary addition li*e D3C and 83C4 3ather it i! ba!ed on binarydi i!ion4

t the !ender !ide6 the data unit to be tran!mitted i! di ided by a #redetermined di i!or

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C ecksu#

Chec*!um i! the error detection method u!ed by u##er layer #rotocol! and i!

con!idered to be more reliable than 83C6 D3C & C3C4

Thi! method ma*e! u!e of chec*!um generator on the !ender !ide &chec*!um chec*er

on the recei er !ide4

t the !ender !ide6 chec*!um generator di ide! the data into e;ual !ubunit! of n bit

length4 Thi! length i! generally of ). bit!4

For e am#le

If the data unit to be tran!mitted i! )A)A)AA) AA)))AA)6 the following #rocedure i!u!ed at the !ender & recei er !ite4

Sender !ite:

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)A)A)AA)

AA)))AA)

)))AAA)A

)A)A)AA) AA)))AA) AAA)))A)

' T Chec*!um

)A)A)AA) !ubunit )

AA)))AA) !ubunit ,

AAA)))A) chec*!um)))))))) !um

AAAAAAAA !um com#liment

3ecei er site:

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Error Correction

Error correction mean! rectifying all the bit! that ha e been altered or changed during the

tran!mi!!ion of data4 Error correction can be done a! follow!:

>y retran!mi!!ion of data from !ender to recei er4

>y the u!e of error correction code at the recei er !ide4

?amming Code

?amming code i! a techni;ue de elo#ed by 3494 hamming for error correction4 Thi! method

correct! the error by finding the !tate at which the error ha! occurred4

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"$o! Contro$

nother im#ortant de!ign i!!ue that occur! in the data lin* layer

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Stop and Wait

In thi! method of flow control6 the !ender !end! a !ingle frame to recei er and wait!

for an ac*nowledgment4

The ne t frame i! !end by !ender only when ac*nowledgment of #re iou! frame i!

recei ed4

Thi! #roce!! of !ending a frame & waiting for an ac*nowledgment conti nue! a! long

a! !ender ha! data to !end4

To end u# the tran!mi!!ion !ender tran!mit! EOT frame

The main ad antage i! it! accuracy4 $e t frame i! tran!mitted only when the

ac*nowledged i! recei ed4

So no chance of frame being lo!t

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Sender Receiver

D !

C"

D !

#O!

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S$iding 'indo!

In !liding window method6 multi#le frame! are !ent by !ender at a time before needing an

ac*nowledgment4 Multi#le frame! !ent by !ource are ac*nowledged by recei er u!ing a !ingle

C" frame4 Sliding window refer! to an imaginary bo e! that hold the frame! on both !ender

and recei er !ide4 It #ro ide! the u##er limit on the number of frame! that can be tran!mitted

before re;uiring an ac*nowledgment4 Frame! may be ac*nowledged by recei er at any #oint

e en window i! not full on recei er !ide4 Frame! may be tran!mitted by !ource e en when

window i! not yet full on !ender !ide4

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. / A ) , + - 0

'indo!

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Error Contro$

Error control function of data lin* layer detect! the error in tran!mitted frame! and retran!mit!

all the erroneou! frame!4 Therefore error control function of data lin* layer hel#! in dealing

with data frame! that are damaged in tran!mit6 data frame lo!t in tran!mit and the

ac*nowledgment frame that are lo!t in tran!mi!!ion4 The method i! u!ed for error control i!

called utomatic 3e#eat 3e;ue!t < 3 =4

utomatic 3e#eat 3e;ue!t < 3 =

If an error i! detected in any frame6 the recei er !end! a negati e a c*nowledgement

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Sto( and 'ait

The !ending de ice *ee#! a co#y of the la!t frame tran!mitted until it recei e! an

ac*nowledgment for that frame4 "ee#ing thi! co#y hel#! the !ender in retran!mi!!ion

of lo!t or damaged frame! later on4

>oth data frame! and C" frame! are numbered alternately A and ) for identification

#ur#o!e4

Criteria u!ed by !to# and wait 3 method:

'amaged data frame!

8o!t data frame!

8o!t ac*nowledged frame!

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'amaged data frame!

If a data frame recei ed by a recei er contain! an error6 it return! $ " frame to the

!ender4 On recei ing $ " frame6 !ender retran!mit! the la!t data frame4

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8o!t data frame!

E ery !ending de ice i! e;ui##ed with timer4

The !ender !tart! thi! timer when it tran!mit! data frame4

If a data frame i! lo!t on it! way6 it will not be recei ed by recei er4 ! a re!ult the

recei er can ne er ac*nowledge it6 #o!iti ely or negati ely4

The !ending de ice wait! for an C" and $ " frame until timer goe! of4

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8o!t ac*nowledgment frame!

9hen any data frame reache! the de!tination6 the recei er ac*nowledge! it either with

C" or with $ "4

If C" or $ " frame returned by the recei er i! lo!t in tran!mit6 !ending de ice

retran!mit the data frame that ha! not been ac*nowledged

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Re)erences

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